Lithographic projection apparatus, gas purging method, device manufacturing method, and purge gas supply system

ABSTRACT

A lithographic projection apparatus is disclosed. The apparatus includes a radiation system for providing a beam of radiation, and a support structure for supporting a patterning device. The patterning device serves to pattern the beam of radiation according to a desired pattern. The apparatus also includes a substrate support for supporting a substrate, a projection system for projecting the patterned beam of radiation onto a target portion of the substrate, and a purge gas supply system. The purge gas supply system includes a purge gas mixture generator that includes a moisturizer that is arranged for adding moisture to a purge gas to generate a purge gas mixture, and a purge gas mixture outlet connected to the purge gas mixture generator for supplying the purge gas mixture to at least part of the lithographic projection apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/623,180, filed on Jul. 21, 2003, the entire content of whichis incorporated herein by reference.

FIELD

The present invention relates to a lithographic projection apparatus, agas purging method, a device manufacturing method, and a purge gassupply system.

BACKGROUND

The term “patterning device” as here employed should be broadlyinterpreted as referring to a device that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such a patterning device include amask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

Another example of a patterning device is a programmable mirror array.One example of such a device is a matrix-addressable surface having aviscoelastic control layer and a reflective surface. The basic principlebehind such an apparatus is that (for example) addressed areas of thereflective surface reflect incident light as diffracted light, whereasunaddressed areas reflect incident light as undiffracted light. Using anappropriate filter, the undiffracted light can be filtered out of thereflected beam, leaving only the diffracted light behind; in thismanner, the beam becomes patterned according to the addressing patternof the matrix-addressable surface. An alternative embodiment of aprogrammable mirror array employs a matrix arrangement of tiny mirrors,each of which can be individually tilted about an axis by applying asuitable localized electric field, or by employing a piezoelectricactuation device. Once again, the mirrors are matrix-addressable, suchthat addressed mirrors will reflect an incoming radiation beam in adifferent direction to unaddressed mirrors; in this manner, thereflected beam is patterned according to the addressing pattern of thematrix-addressable mirrors. The required matrix addressing can beperformed using suitable electronic devices. In both of the situationsdescribed hereabove, the patterning device can include one or moreprogrammable mirror arrays. More information on mirror arrays as herereferred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891and 5,523,193, and PCT Patent Publication Nos. WO 98/38597 and WO98/33096, which are incorporated herein by reference. In the case of aprogrammable mirror array, the support structure may be embodied as aframe or table, for example, which may be fixed or movable as required.

Another example of a patterning device includes a programmable LCDarray. An example of such a construction is given in U.S. Pat. No.5,229,872, which is incorporated herein by reference. As above, thesupport structure in this case may be embodied as a frame or table, forexample, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table. However, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. including one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion in one go; such an apparatus is commonly referred toas a wafer stepper or step-and-repeat apparatus. In an alternativeapparatus—commonly referred to as a step-and-scan apparatus—each targetportion is irradiated by progressively scanning the mask pattern underthe projection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction; since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat. No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, as appropriate, for example, for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.

Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, both incorporated herein by reference.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion techniques are well known in the artfor increasing the numerical aperture of projection systems.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultraviolet(UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm)and extreme ultra-violet (EUV) radiation (e.g. having a wavelength inthe range 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

In general, surfaces of components present in a lithographic projectionapparatus become contaminated during use, even if most of the apparatusis operated in vacuum. In particular the contamination of opticalcomponents in the lithographic projection apparatus, such as mirrors,has an adverse effect on the performance of the apparatus, because suchcontamination affects the optical properties of the optical components.

In the art, it is known to reduce contamination of optical components ofa lithographic projection apparatus by purging a space of thelithographic projection apparatus in which such a component is locatedwith an ultra high purity gas, which is from hereon referred to as apurge gas. The purge gas prevents contamination of the surface, forexample molecular contamination with hydrocarbons.

A drawback of this known method is that the purge gas may have anadverse effect on the activity of chemicals used in the lithographicprojection process. In particular, it is found by the applicant thatsome types of radiation-sensitive material (resist), in particularresists sensitive to ultra-violet radiation and acetal-basephoto-resists, do not function properly in an environment provided withthe purge gas. Experiments performed by the applicant have revealed thatthese resists require a moisture, e.g. water vapour, to develop.

Furthermore, the purge gas may have an effect on the performance ofmeasurement devices present in the lithographic projection apparatus,such as interferometric instruments for example. It is found by theapplicant that because of the lack of moisture, the purge gas affectsthe refractive index and thereby changes the outcome of interferometricmeasurements as well.

However, such a moisture is not present in the purge gas used in theknown methods. Thus, contamination cannot be reduced using the knownpurge gas supply system for these types of resists.

Although also a clean gas, strictly speaking the gas used for gasbearings in e.g. an immersion lithography apparatus differs from a purgegas in that e.g. the purity requirements are less strict, and in that itis provided at a much higher pressure. This high pressure gas flowprovides for a stable and small gap between a surface of the substrateand the “shower head” of the immersion lithography apparatus, therebyreducing the likelihood of collision between the shower head and thesubstrate. However, the same problem as regards affectinginterferometric measurements because of lack of moisture also applies toair bearing gas.

SUMMARY

It is a general aspect of the present invention to provide an improvedlithographic projection apparatus, and in particular a lithographicprojection apparatus in which contamination can be reduced with a purgegas without affecting the development of the resist.

It is a further aspect of the present invention to provide an improvedlithographic projection apparatus in which purge gas and gas used in gasbearings do not affect interferometric measurements.

The invention therefore provides a lithographic projection apparatusthat includes a radiation system for providing a beam of radiation;, anda support structure for supporting a patterning device. The patterningdevice serves to pattern the beam of radiation according to a desiredpattern. The apparatus also includes a substrate table for holding asubstrate, a projection system for projecting the patterned beam onto atarget portion of the substrate, and at least one purge gas supplysystem for providing a purge gas to at least part of the lithographicprojection apparatus. The purge gas supply system includes a purge gasmixture generator that includes a moisturizer arranged for addingmoisture to a purge gas. The purge gas mixture generator is arranged forgenerating a purge gas mixture. The purge gas mixture includes at leastone purge gas and moisture. The purge gas generator also includes apurge gas mixture outlet connected to the purge gas mixture generatorfor supplying the purge gas mixture to the part of the lithographicprojection apparatus.

The invention also provides a method for providing a purge gas to atleast part of a lithographic apparatus. The method includes addingmoisture to a purge gas to generate a purge gas mixture, and supplyingthe purge gas mixture to at least a part of the lithographic projectionapparatus. Thus, chemicals used in the lithographic projection apparatusare not affected by the purge gas.

According to a further aspect of the invention, a device manufacturingmethod is provided. The method includes projecting a patterned beam ofradiation onto a target portion of a layer of radiation-sensitivematerial on a substrate, adding moisture to a purge gas to generate apurge gas mixture, and supplying the purge gas mixture near a surface ofthe substrate.

The invention further provides a purge gas supply system for alithographic apparatus. The system includes a purge gas mixturegenerator that includes a moisturizer arranged for adding moisture to apurge gas. The purge gas mixture generator is arranged for generating apurge gas mixture. The purge gas mixture includes at least one purginggas and the moisture. The system also includes a purge gas outlet forsupplying the purge gas mixture to at least part of the lithographicprojection apparatus.

Further details, aspects and embodiments of the invention will bedescribed, by way of example only, with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an example of an embodiment of a lithographicprojection apparatus according to invention.

FIG. 2. shows a side view of an EUV illuminating system and projectionoptics of a lithographic projection apparatus according to theinvention.

FIG. 3 schematically shows a circuit diagram of an example of a purgegas supply system according to the invention.

FIG. 4 schematically shows a moisturizer device suitable for in theexample of FIG. 3.

DETAILED DESCRIPTION

FIG. 1 schematically depicts an example of an embodiment of alithographic projection apparatus 1 according to the invention. Theapparatus 1 typically includes: a radiation system Ex, IL, for supplyinga projection beam PB of radiation (e.g. UV or EUV radiation). In thisparticular case, the radiation system also includes a radiation sourceLA; a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g. a reticle), and connected to a first positioningdevice PM for accurately positioning the mask with respect to item PL; asecond object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to a second positioning device PW for accuratelypositioning the substrate with respect to item PL; and a projectionsystem (“lens”) PL (e.g. a mirror group) for imaging an irradiatedportion of the mask MA onto a target portion C (e.g. including one ormore dies) of the substrate W.

As here depicted, the apparatus is of a reflective type (i.e. has areflective mask). However, in general, it may also be of a transmissivetype, for example (with a transmissive mask). Alternatively, theapparatus may employ another kind of patterning device, such as aprogrammable mirror array of a type as referred to above.

The source LA (e.g. a Hg lamp, an excimer laser, an undulator or wigglerprovided around the path of an electron beam in a storage ring orsynchrotron, a laser-produced plasma source or otherwise) produces abeam of radiation. This beam is fed into an illumination system(illuminator) IL, either directly or after having traversed aconditioning device, such as a beam expander Ex, for example. Theilluminator IL may include an adjusting device AM for setting the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally include various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors); this latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having been selectively reflected by the mask MA, the beam PBpasses through the projection system PL, which focuses the beam PB ontoa target portion C of the substrate W. With the aid of the secondpositioning device PW (and an interferometric measuring device IF), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning device PM can be used to accurately position the maskMA with respect to the path of the beam PB, e.g. after mechanicalretrieval of the mask MA from a mask library, or during a scan. Ingeneral, movement of the object tables MT, WT will be realized with theaid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which are not explicitly depicted in FIG. 1.However, in the case of a wafer stepper (as opposed to a step-and-scanapparatus) the mask table MT may just be connected to a short strokeactuator, or may be fixed. Mask MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected in one go (i.e. a single “flash”) ontoa target portion C. The substrate table WT is then shifted in the xand/or y directions so that a different target portion C can beirradiated by the beam PB; and

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (typically,M=1/4 or 1/5). In this manner, a relatively large target portion C canbe exposed, without having to compromise on resolution.

FIG. 2 shows a projection system PL and a radiation system 2 which canbe used in the example of a lithographic projection apparatus 1 ofFIG. 1. The radiation system 2 includes an illumination optics unit 4.The radiation system may also include a source-collector module orradiation unit 3. The radiation unit 3 is provided with a radiationsource LA which may be formed by a discharge plasma. The radiationsource LA may employ a gas or vapor, such as Xe gas or Li vapor in whicha very hot plasma may be created to emit radiation in the EUV range ofthe electromagnetic spectrum. The very hot plasma is created by causinga partially ionized plasma of an electrical discharge to collapse ontothe optical axis O. Partial pressures of 0.1 mbar of Xe, Li vapor or anyother suitable gas or vapor may be required for efficient generation ofthe radiation. The radiation emitted by radiation source LA is passedfrom the source chamber 7 into collector chamber 8 via a gas barrierstructure or “foil trap” 9. The gas barrier structure includes a channelstructure such as, for example, described in detail in European patentapplications EP-A-1 233 468 and EP-A-1 057 079, which are incorporatedherein by reference.

The collector chamber 8 includes a radiation collector 10 which can beformed by a grazing incidence collector. Radiation passed by collector10 is reflected off a grating spectral filter 11 to be focused in avirtual source point 12 at an aperture in the collector chamber 8. Fromchamber 8, the projection beam 16 is reflected in illumination opticsunit 4 via normal incidence reflectors 13, 14 onto a reticle or maskpositioned on reticle or mask table MT. A patterned beam 17 is formedwhich is imaged in projection system PL via reflective elements 18, 19onto a wafer stage or substrate table WT. More elements than shown maygenerally be present in illumination optics unit 4 and projection systemPL.

As is shown in FIG. 2 the example of a lithographic projection apparatus1 according to an embodiment of the invention of FIG. 1 includes a purgegas supply system 100 according to the invention. Purge gas outlets130-133 of the purge gas supply system 100 are positioned in theprojection system PL and the radiation system 2 near the reflectors13,14 and the reflective elements 18,19, as is shown in FIG. 2. However,if so desired other parts of the apparatus may likewise be provided witha purge gas supply system according to an embodiment of the invention.For example, a reticle and one or more sensors of the lithographicprojection apparatus may be provided with a purge gas supply systemaccording to the invention.

In FIGS. 1 and 2, the purge gas supply system 100 is positioned insidethe lithographic projection apparatus 1 and the purge gas supply system100 can be controlled in any manner suitable for the specificimplementation using any convenient means outside the apparatus 1.However, it is likewise possible to position at least some parts of thepurge gas supply system 100 outside the lithographic projectionapparatus 1, such as for example the purge gas mixture generator 120 orotherwise.

FIG. 3 shows a practical example of a purge gas supply system 100.However, a similar system as shown in FIG. 3 may likewise be utilized inconditioning gas used in gas bearings in e.g. an immersion lithographyapparatus. In the example of FIG. 3, a purge gas inlet 110 is connectedto a, not shown, purge gas supply apparatus which supplies a dry gaswhich is substantially without moisture, such as, for example, apressurised gas supply circuit, a cylinder with compressed dry air orotherwise. The dry gas is fed through the purge gas mixture generator120. In the purge gas mixture generator 120 the dry gas is purifiedfurther, as is explained below in more detail. Further, the purge gasmixture generator 120 includes a moisturizer 150 which adds a moistureto the dry gas, for some of the purge gas outlets 130-132. In theexample of FIG. 3, the moisturizer 150 is connected a single purge gasoutlet 130. The other purge gas outlets 131,132 are not connected to themoisturizer 150. Thus, at the purge gas outlet 130, a purge gas mixtureincluding the purge gas and moisture is presented, whereas at the otherpurge gas outlets 131,132 only the dry purge gas is presented. Therebythe purge gas mixture may be provided only near surfaces provided withchemicals which require a moisture, such as the wafer table WT in theexample of FIG. 1, whereas other parts of the lithographic projectionapparatus 1 can be provided with a ‘dry’ purge gas, i.e, withoutmoisture.

Furthermore, because the moisture is added to a purge gas, properties ofthe purge gas mixture, such as the relative humidity or purity of themoisture, can be controlled with a good accuracy. Also, because of themoisturizer the system is flexible, because the amount of moisturepresent in the purge gas mixture may easily be adjusted by adding moreor less moisture to the purge gas.

The purge gas mixture generator 120 in the example of FIG. 3 includes,in a flow direction and that order: a purifier apparatus 128, a flowmeter 127, a valve 125, a reducer 129, a heat exchanger 126 and amoisturizer 150.

In the example of FIG. 3, compressed dry air (CDA) from a, not shown,CDA source is supplied to the purifier apparatus 128 via the purge gasinlet 110. The CDA is purified by the purifier 128. The purifier 128includes two parallel flow branches 128A,128B each including, in theflow direction and that order: an automatic valve 1281,1282 and aregenerable purifier device 1283,1284. The regenerable purifier devices1283,1284 are each provided with a heating element to heat and therebyregenerate the respective purifier device 1283,1284. The flow branchesare connected downstream of the purifier devices 1283,1284 to a shut-offvalve 1285 which is controlled by a purity sensor 1286.

Because of the regenerable purifiers, the system can be used for a longtime period by regenerating the purifiers in case they become saturatedwith the compounds removed from the purge gas. The regenerable purifiersmay be of any suitable type, such as for example a, known as such,regenerable filter which removes contaminating compounds or particlesout of a gas by means of a physical process, such as adsorption,catalysis or otherwise, as opposed to the, non regenerable, chemicalprocesses occurring in a charcoal filter, for example. In general, aregenerable purifier does not contain organic material and theregenerable purifiers may for example contain a material suitable forphysical binding a contaminant of the purge gas, such as for example:metals, zeolite, titanium oxides, gallium or palladium compounds, orotherwise.

In the example of FIG. 3, the purifier devices 1283,1284 are alternatelyput in a purifying state in which the CDA is purified and a regeneratingstate. In the regenerating state the purifier device is regenerated bymeans of the respective heating element. Thus, for example, while thepurifier device 1283 purifies the CDA, the purifier device 1284 isregenerated. The purifier 128 can thus operate continuously whilemaintaining a constant level of purification.

The automatic valves 1281,1282 are operated in correspondence with theoperation of the corresponding purifier device 1283,1284. Thus, when apurifier device 1283,1284 is regenerated, the corresponding valve1281,1282 is closed, while when a purifier device 1283,1284 is used topurify, the corresponding valve is open.

The purified CDA is fed through the shut-off valve 1285 which iscontrolled by the purity sensor 1286, which is known per se and for thesake of brevity is not described in further detail. The purity sensor1286 automatically closes the shut-off valve 1285 when the purity of thepurified CDA is below a predetermined threshold value. Thus,contamination of the lithographic projection apparatus 1 with a purgegas with insufficient purity levels is prevented automatically.

The flow of purified CDA can be monitored via the flow meter 127. Viathe valve 125 the flow can be shut-off manually. The reducer 129provides a stable pressure at the outlet of the reducer, thus a stablepurge gas pressure is provided to restrictions (via the heat exchanger126).

The heat exchanger 126 provides a constant purified CDA temperature. Theheat exchanger 126 extracts or adds heat to the purified CDA in order toachieve a gas temperature which is suitable for the specificimplementation. In a lithographic projection apparatus, for example,stable processing conditions are required and the heat exchanger maythus stabilize the temperature of the purified CDA to have a gastemperature which is constant over time. Suitable conditions for thepurge gas at the purge gas outlets, for example, are found to be: a flowof 20-30 standard litres per minute, and/or a temperature of the purgegas of about 22 degrees Celsius and/or a relative humidity in the rangeof 30-60%. However, the invention is not limited to these conditions andother values for these parameters may likewise be used in a systemaccording to the invention.

The heat exchanger 126 is connected via restrictions 143-145 to thepurge gas outlets 130-132. The restrictions 143-145 limit the gas flow,such that at each of the purge gas outlets 130-132 a desired, fixedpurge gas flow and pressure is obtained. A suitable value for the purgegas pressure at the purge gas outlets is for example 100 mbar. It islikewise possible to use adjustable restrictions to provide anadjustable gas flow at each of the purge gas outlets 130-132.

The moisturizer 150 is connected downstream from the heat exchangerbetween the restriction 143 and the purge gas outlet 130. The purge gasoutlet 130 is provided in the example of FIGS. 1 and 2 near the wafertable WT. The moisturizer 150 adds a moisture to the purified CDA andthus provides a purge gas mixture to the outlet 130. In this example,only at a single outlet a purge gas mixture is discharged. However, itis likewise possible to discharge a purge gas mixture to two or morepurge gas outlets, for example by connecting a multiple of purge gasoutlets to separate moisturizers or connecting two or more outlets tothe same moisturizer. It is likewise possible to provide a moisturizerat a different position in the purge gas mixture generator than is shownin FIG. 3. For example, the moisturizer 150 may be placed between thepurge gas mixture generator 120 and the valve 143 instead of between thevalve 143 and the purge gas outlet 130. The moisturizer 150 operates asa restriction as well and if so desired, the restriction 143 connectedto the moisturizer 150 may be omitted.

In an alternative embodiment of a purge gas supply system according tothe invention, an additional heat exchanger (not shown) is provided atthe purge gas outlet 130 for a better temperature control of the purgegas mixture.

The moisturizer 150 in FIG. 3 may for example be implemented as theexample of FIG. 4, however the moisturizer 150 may likewise beimplemented differently, and for example include a vaporiser whichvaporises a fluid into a flow of purge gas or otherwise.

The moisturizer 150 shown in FIG. 4 includes a liquid vessel 151 whichis filled to a liquid level A with a liquid 154, such as high puritywater for example. A gas inlet 1521, from hereon referred to as the wetgas inlet 1521, is placed mounding submerged in the liquid 154, that isbelow the liquid level A. Another gas inlet 1522, from hereon referredto as the dry gas inlet 1522, is placed mounding above the liquid levelA, i.e. in the part of the liquid vessel 151 not filled with the liquid154. A gas outlet 153 connects the part of the liquid vessel 153 abovethe liquid 154 with other parts of the purge gas supply system 100. Apurge gas, e.g. purified compressed dry air, is fed into the liquidvessel 151 via the wet gas inlet 1521. Thus, bubbles 159 of purge gasare generated in the liquid 154. Due to buoyancy forces, the bubbles 159travel upwards after mounding in the liquid 154, as indicated in FIG. 4with arrow B. During this upwards travelling period, moisture from theliquid 154 enters the bubbles 159, for example due to diffusiveprocesses or otherwise. Thus, the purge gas in the bubbles 159 is mixedwith a moisture. At the surface of the liquid i.e. at the liquid levelA, the bubbles 159 supply their gaseous content to the gas(es) presentin the liquid vessel 151 above the liquid 154. The resulting purge gasmixture is discharged from the vessel via the gas outlet 153.

In the example of FIG. 4, the wet gas inlet 1521 is a tubular elementwith an outside end connected outside the liquid vessel 151 to a, notshown, purge gas supply device, such as the purge gas mixture generator120 of FIG. 3, for example. The wet gas inlet 1521 is provided with afilter element 1525 with small, e.g. 0.5 micron, passages at an insideend which is positioned in the inside of the liquid vessel 151. Thefilter element 1525 is at least partially, (in this example entirely)placed in the liquid 154. Thus, the wet gas inlet 1521 generates a largeamount of very small bubbles of purge gas. Because of their small size,in this example about 0.5 micron however other suitable dimensions maylikewise be used, the bubbles 159 are moisturized to saturation in arelatively short time period, i.e. a relatively short travellingdistance through the liquid 154.

The dry gas inlet 1522 is provided with a filter element 1524 similar tothe filter element of the wet gas inlet 1521. Thereby, the gas flowthrough the wet gas inlet 1521 and the dry gas inlet 1522 issubstantially similar, and the amount of moisture in the purge gasmixture is substantially half the amount of moisture in the bubbles 159at the moment the bubbles 159 leave the liquid 154. That is, if thebubbles 159 are saturated with moisture, i.e. 100% relative humidity(Rh), the purge gas mixture has a 50% Rh. However, it is likewisepossible to provide in a different ratio of gas flowing into the liquidvessel via the wet gas inlet 1521 and the dry gas inlet 1522respectively and thereby adjust the relative humidity between 0-100% Rh.

It is found by the applicant that specifically a purge gas mixture witha relative humidity above or equal to 20%, such as equal or more than25%, provides good results with respect to the performance ofphoto-resists. Furthermore, it is found that a purge gas mixture with arelative humidity equal or above 25% and below 70%, such as 60%, has agood preventive effect with respect to the accuracy of measurementsystems in the lithographic projection apparatus. Furthermore, it wasfound that a humidity, e.g. about 40%, which is similar to the humidityin the space surrounding the lithographic projection apparatus, e.g. thecleanroom, provides optimal results.

The gas outlet 153 is provided at its inside end with a fine-meshed,e.g. 0.003 micron, filter 1526 which filters particles and smalldroplets out of the gas flowing out of the liquid vessel 151. Thus,contamination of the surface to which the purge gas mixture is suppliedby such particles is prevented.

In the example of FIG. 4, the relative amount of moisture in the purgegas mixture can be controlled in an uncomplicated manner, in differentways. For example parameters of the liquid vessel can be controlled.Also, for example, the amount of purge gas without moisture brought intothe vessel 151 via the dry gas inlet 1522 relative to the amount ofpurge gas with moisture generated via the wet gas inlet 1521 can becontrolled. The controlled parameters of the liquid vessel 151 may forexample be one or more of: the inside temperature, flow, pressure,residence time of the purge gas in the liquid.

Temperature is known to have an effect on the saturation amount ofmoisture that can be present in a gas, for example. To control thetemperature, the liquid vessel 151 may be provided with a heatingelement which is controlled by a control device in response to atemperature signal representing a temperature inside the liquid vesselprovided by a temperature measuring device, for example.

The residence time of the bubbles in the liquid 154 can be changed byadjusting the position at which the gas bubbles are inserted in theliquid via the wet gas inlet 1521. For example when the filter 1525 ispositioned further into the liquid 154, the distance the bubbles have totravel to the liquid level A is increased and hence the residence timeincreases as well. The longer the gas bubbles are present in the liquid154, the more moisture can be absorbed into the gas. Thus, by changingthe residence time the humidity of the gas can be adapted.

The moisturizer device 150 of FIG. 4 is further provided with a controldevice 157 via which the amount of moisture in the purge gas mixture canbe controlled. As shown in FIG. 4, the control device 157 is connectedwith a moisture control contact 1571 to a control valve 1523 in the drygas inlet 1522 via which the flow rate of the purge gas supplied to thedry inlet 1522 can be controlled and therefore the amount of dry purgegas relative to the amount of moisturized gas.

A humidity sensor (not shown) at the gas outlet 153, communicativelyconnected to the control device 157, provides to the control device 157the humidity information of the purge gas mixture at the gas outlet 153.This information can be used by the control device 157 to adjust atleast one of the parameters to control the amount of moisture in thepurge gas mixture.

The control device 157 further controls the amount of liquid 154 presentin the liquid vessel 151. The control device 157 is connected with aliquid control contact 1572 to a control valve 1561 of a liquid supply156 and with an overflow contact 1573 to a control valve 1531 of the gasoutlet 153. A liquid level measuring device 158 is communicativelyconnected to the control device. The liquid level measuring device 158provides a liquid level signal to the control device 157 whichrepresents a property of the liquid level in the liquid vessel 151. Thecontrol device 157 operates the control valve 1561 and a control valve1531 in response to the liquid level signal.

In this example, the liquid level measuring device 158 includes threefloat switches 1581-1583 positioned at suitable, different, heights withrespect to the bottom of the liquid vessel 151. A lowest float switch1581 is positioned nearest to the bottom. The lowest float switch 1581provides an empty signal to the control device 157 when the liquid levelA is at or below the lowest float switch 1581. In response to the emptysignal, the control devices 157 opens the control valve 1561 andautomatically liquid is supplied to the vessel.

The float switch 1582 in the middle provides a full signal in case theliquid level A reaches the height of this flow switch 1582. The controldevice 157 closes the control valve 1561 in response to the full signaland thereby turns off the liquid supply.

A top float switch 1583 is positioned furthest away from the bottom. Thetop float switch 1583 provides an overfill signal to the control device157 in case the liquid level A is at or above the top float switch 1581.In response to the overfill, the control device 157 shuts off thecontrol valve 1531 of the gas outlet 153 to prevent leakage of theliquid into other parts of the lithographic projection apparatus 1.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design alternatives without departing from the scope of theappended claims. In particular, although the invention has mainly beendescribed in conjunction with a purge gas supply system, it is to beunderstood, however, that the invention may also be applied in providinga moisturized gas to be used in a high-pressure gas bearing in e.g. animmersion lithography apparatus.

1. A lithographic projection apparatus comprising: a radiation systemfor providing a beam of radiation; a support structure for supporting apatterning device, the patterning device serving to pattern the beam ofradiation according to a desired pattern; a substrate table for holdinga substrate; a projection system for projecting the patterned beam ontoa target portion of the substrate; and at least one purge gas supplysystem for providing a purge gas mixture to at least part of thelithographic projection apparatus, said purge gas supply systemcomprising a purge gas mixture generator comprising a moisturizerarranged for adding moisture to a purge gas to generate the purge gasmixture; and a purge gas mixture outlet connected to the purge gasmixture generator for supplying the purge gas mixture to said at leastpart of the lithographic projection apparatus.
 2. A lithographicprojection apparatus as claimed in claim 1, wherein the moisturizercomprises a liquid vessel with at least one gas inlet and a gas outlet,said gas inlet and gas outlet being connected to each other via amoisturizing connection, such that in case the purge gas flows throughthe moisturizing connection, said purge gas is fed through a liquidpresent in the liquid vessel and said purge gas is moisturized.
 3. Alithographic projection apparatus as claimed in claim 2, furthercomprising a dry gas inlet connected to the gas outlet for mixing anon-moisturized purge gas with the purge gas fed through the liquid toobtain said purge gas mixture.
 4. A lithographic projection apparatus asclaimed in claim 2, wherein the moisturizing connection is a saturatingconnection for feeding said purge gas through said liquid such that saidpurge gas is moisturized to saturation with said moisture.
 5. Alithographic projection apparatus as claimed in claim 2, furthercomprising a controller connected to the liquid vessel for controllingat least an amount of moisture present in the purge gas mixture.
 6. Alithographic projection apparatus as claimed in claim 1, wherein thepurge gas mixture generator further comprises a regenerable filter forfiltering at least one undesired component out of the purge gas, themoisture, and/or the purge gas mixture.
 7. A lithographic projectionapparatus as claimed in claim 6, comprising two regenerable filtersconnected in parallel, said filters constructed and arranged to beregenerated in an alternating manner for allowing a continuousfiltering.
 8. A lithographic projection apparatus as claimed in claim 1,wherein the purge gas supply system further comprises a purge gas outletarranged for providing said purge gas substantially without moisture toanother part of the lithographic projection apparatus.
 9. A lithographicprojection apparatus as claimed in claim 1, wherein said moistureincludes water vapor.
 10. A lithographic projection apparatus as claimedin claim 9, wherein said purge gas mixture contains between about 20% Rhwater vapor, and about 70% Rh water vapor.
 11. A method for forproviding a purge gas mixture to at least part of a lithographicprojection apparatus comprising a radiation system for providing a beamof radiation; a support structure for supporting a patterning device,the patterning device serving to pattern the beam of radiation accordingto a desired pattern; a substrate table for holding a substrate; and aprojection system for projecting the patterned beam onto a targetportion of the substrate, said method comprising: adding moisture to apurge gas to generate said purge gas mixture; and supplying the purgegas mixture to at least a part of the lithographic projection apparatus.12. A device manufacturing method comprising: projecting a patternedbeam of radiation onto a target portion of a layer ofradiation-sensitive material on a substrate; adding moisture to a purgegas to generate a purge gas mixture; and supplying the purge gas mixturenear a surface of the substrate.
 13. A purge gas supply system forproviding a purge gas to at least part of a lithographic projectionapparatus, said purge gas supply system comprising: a purge gas mixturegenerator comprising a moisturizer arranged for adding moisture to apurge gas, said purge gas mixture generator being arranged forgenerating a purge gas mixture comprising at least one purging gas andsaid moisture; and a purge gas outlet for supplying the purge gasmixture to said at least part of the lithographic projection apparatus.